1. Field of the Invention
The invention relates to an exterior element sensor for detecting elements such as rain, snow, water, liquids, dirt and other debris on a window. In particular, the invention relates to an exterior element sensor which can be mounted on a surface such as a window, an automobile windshield, or other structure that loses its field of vision when enough raindrops or other exterior elements contact its surface. The exterior element sensor can be employed to automatically operate wipers when a predetermined amount of exterior elements such as raindrops, snow, water, debris, etc. are detected on the window or windshield. Together with the exterior element sensor of this type, a light sensor is often provided to automatically turn on/off headlights or other devices in response to variations in surrounding brightness at sunset or on access to a tunnel and evacuation therefrom, during rain, snow, or other circumstances.
2. Description of the Related Art
A typical configuration for an exterior element sensor of this type in the art is shown in FIG. 6. In this case, an incident optical element 11a is attached to an interior surface of a window W such as a front window via a right prism with an apex of 90° facing upward, using a transparent adhesive 14 such as an epoxy resin. The configuration yields a surface angled at 45° to the window surface. Light from a light-emitting element such as an LED is emitted into the angled surface at a right angle thereto.
Thus, the light coming into the surface of the window W from the incident optical element can pass into the window W without suffering refraction due to a difference between refractive indexes of the atmosphere and the window. Even when the light reaches an exterior surface Wo of the window, the angle of 45° (which is larger than the critical angle) can be held. Therefore, in the exterior surface Wo of the window, total reflection arises at an inner surface of the window.
When total reflection occurs at the inner surface of the window in this way, an angle of incidence is the same as an angle of reflection. Accordingly, the light that is totally reflected from the exterior surface Wo of the window is folded back and reaches either the interior surface Wi of the window W or a high refractive index plane that is in parallel with the interior surface from which it is totally reflected. Thereafter, similar total reflection is repeated between the exterior surface and the interior surface, alternately. Thus, the light is confined within the window W.
Provided therefore is an emissive optical element 11b. This emissive optical element 11b includes a right prism, which is configured the same as that of the incident optical element 11a and is similarly adhered onto the inner surface of the window W with a transparent resinous adhesive, for example. The place of adhesion is a location on the interior surface Wi of the window, to which the light coming into the incident optical element 11a returns, after it is totally reflected at least once from the exterior surface Wo of the window W. In general, the incident optical element 11a and the emissive optical element 11b are integrated into a sensing optical element 11 in consideration of the thickness and the critical angle of the window W, and both can be arranged in place in one adhesion work.
Thus, the light coming into the incident optical element 11a and totally reflected at least once from the exterior surface Wo of the window can exit from the emissive optical element 11b. The amount of this light is always observed at a photoreceptor 13. In this case, if raindrops are attached to the exterior surface Wo of the window during a rainfall, for example, since the window and the raindrops (water) can have approximately the same refractive indexes, shape variations may occur in the reflecting surface. In this state, the resultant leakage of light and diffused reflection reduces the amount of light observed at the photoreceptor 13, which can thus detect the amount of raindrops, or the rainfall. Though it is not shown in FIG. 6, in vehicles in which detected rainfall automatically controls wipers, an exterior light sensor may often be provided separately to turn on/off the light depending on the external brightness (Please see Japanese Patent Document JP-A 2001-66246, for example).
In the above conventional configuration, however, the raindrop sensor requires a light source. As shown in FIG. 1, the exterior element sensor is not configured to consider the joint use of sensing exterior light, of the related art (see Japanese Patent Document JP-A 2001-66246). Therefore, the exterior element sensor and the exterior light sensor are provided separately. Accordingly, the sensors are often provided at two locations on the front of the windshield, which blocks a large field of vision, resulting in a problem in that the driver of the vehicle experiences an increase in lost field of vision.
Providing the exterior element sensor and the exterior light sensor separately requires respective special-purpose lenses, cases, and circuit components, which increases the number of components and the steps of assembling. Thus, increasing management and production problems, which inevitably elevates the overall cost of production.
Furthermore, when the incident optical element 11a and the emissive optical element 11b are adhered onto the window W, a positional relation between both requires accuracy. Therefore, as shown in FIG. 6, a prism body 11c must be formed in the shape of two right prisms connected at a certain interval to each other, requiring a high process technology and further increasing the cost.